Environmentally harmful species in the exhaust gas emitted from an internal combustion engine, such as hydrocarbons (HC), carbon monoxide (CO), particulate matters (PM), and nitric oxides (NOx) are regulated species that need to be removed from the exhaust gas. In lean combustion engines, due to the effects of large amount oxygen excess, passive means without extra dosing agents, such as that using a three-way catalyst, normally are not able to effectively remove the oxidative specie NOx, as that in most of spark-ignition engines. To reduce NOx in lean combustion engines, a variety of active means with reducing agents (reductants) being dosed in exhaust gas are developed. In these technologies, typically the reductant is metered and injected into the exhaust gas, and the result mixture flows into a SCR catalyst, where the reductant selectively reacts with NOx generating non-poisonous species, such as nitrogen, carbon dioxide, and water.
A variety of reductants, such as ammonia (NH3), HC, and hydrogen (H2) can be used in SCR systems. Among them, ammonia SCR is used most broadly due to high conversion efficiency and wide temperature window. Ammonia can be dosed directly. However, due to safety concerns and difficulties in handling pure ammonia, normally urea solution, which is also called reductant, is used in ammonia SCR systems. Urea can be converted to ammonia in exhaust gas through thermolysis and hydrolysis.
Typically, in a SCR control system, the required ammonia dosing rate is calculated in an ECU (Engine Control Unit) or in a DCU (Dosing Control Unit) in response to a few parameters, such as engine operating parameters, exhausts gas temperature, exhaust gas flow rate, and engine out NOx level. In these parameters, the engine operating parameters, including engine fueling rate and engine speed, which are used in both estimating key factor values in SCR controls and diagnostics and enable conditions in SCR system diagnostics, are critical. For example, in some engine systems, especially engine systems without using EGR (Exhaust Gas Recirculation), both of the engine out NOx level and exhaust gas flow rate can be accurately estimated with the engine fueling rate and engine speed, while in engine systems with EGR, the engine fueling rate and engine speed are also key factors in the estimation. The exhaust flow rate and engine out NOx level are key parameters in both of SCR controls and diagnostics, while in a few SCR diagnostics, such as NOx sensor rationality diagnostics, the engine fueling rate is used as a key enable condition in comparing engine out NOx level and NOx sensor reading.
Normally the engine operating parameter values are obtained from engine controls. However, the engine operating parameter values are not always available, and in some engine systems, even they are available, their applications are limited due to the limits of the system structure. For example, in applications with mechanically controlled engines, e.g. in a vehicle retrofit, ECU and the engine operating parameter values are not available since engine fueling is controlled mechanically. In engine systems with multiple exhaust branches, e.g. in a high horse-power engine system, even the overall exhaust flow rate can be estimated with the engine fueling rate and engine speed, exhaust flow rate in each branch is not available. In these applications, to control the SCR system and diagnosing issues in the system, either more sensors, such as engine speed sensors and throttle position sensors, are installed in the engine system for obtaining the engine operating parameters, or more assumptions are used in estimation, e.g., assuming exhaust flow is equally distributed in each exhaust branch. Installing new sensors in an engine system changes system structure, causing reliability issues, while more assumptions deteriorate control performance and diagnosis capabilities. Moreover, when sensors are installed in the engine system, different engine types and applications require different sensor types, resulting in high system cost and engineering cost.
To reliably control a SCR system in an engine system without engine operating parameter values, a primary object of the present invention is to provide an apparatus to obtain key parameter values in a SCR control and diagnostic system without using engine operating parameters, so that the SCR control and diagnostic system is able to work without relying on them.
A further object of the present invention is to provide an apparatus to obtain key parameter values of a SCR control system only using sensors installed in the SCR control system, so that no further assumption is required for multiple branch systems.
Another object of the present invention is to provide a multi-functional sensing apparatus in a SCR control system, which not only provides key parameter values of the SCR control system, but also facilitates SCR controls.
Yet another object of the present invention is to provide a multi-functional sensing apparatus in a SCR control system, which not only provides key parameter values of the SCR control system, but also provides engine operating parameter values, which can be further used for diagnosing issues in an engine system.